1. Field of the Invention
The present invention relates to a toner for use in developing latent electrostatic images formed in the electrostatic photocopying process performed by a photocopier, a facsimile machine, a printer, etc and also relates to a fixing device and an image forming apparatus using this toner.
2. Discussion of the Background
An image forming apparatus is typically used for image forming methods such as electrophotography, electrostatic recording and electrostatic printing. The image forming methods typically include the following processes:    (1) A latent electrostatic image formed on an image bearing member such as photoreceptors or dielectric materials is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (2) the toner image is transferred on a receiving material such as recording papers optionally via an intermediate transfer medium (transfer process); and    (3) the toner image is fixed on the receiving material upon application of heat and/or pressure, or the like (fixing process).
In the fixing process mentioned above, a pair of fixing members having a roller form or a belt form including a heater in its interior fix the toner on a recording material upon application of heat and pressure while pinching the receiving material. When the heating temperature is too high, the toner excessively fuses, which causes a problem in that the fused toner is attached to the fixing member, i.e., hot offset problem. When the heating temperature is too low, the toner does not sufficiently fuse, and thus the fixing is not sufficient. To save energy and reduce the size of an image forming apparatus, a toner having a high hot offset temperature (anti hot offset property) and low fixing temperature (low fixing temperature property) is desired. In addition, the toner is necessary to be free from blocking when the toner is in store or an atmosphere in the apparatus (high temperature preservability).
Especially, to obtain good images, full color photocopiers and full color printers require good gloss and color mixture properties. Therefore, the toner is necessary to have a low viscosity. Consequently, the toners used are polyester toner binders having a sharp melt property. However, these toners easily cause hot offset and thus silicone oil, etc., is usually applied to the fixing member included in a full color image forming apparatuses. To apply such silicone oil to the fixing member, a device such as an oil tank and an oil application device is required. Consequently the image forming apparatus is jumboized and complicated. Further, such oil causes deterioration of the fixing member and requires maintenance on a regular interval basis. In addition, oil attachment to receiving materials such as plain copy papers and transparent films is inevitable. The color tone can be degraded by oil especially when the oil is attached to a transparent film.
Therefore, instead of applying oil to the fixing member, a release agent is typically added to a toner to avoid that toner attachment problem. However, the release effect of a release agent greatly depends on the dispersion state thereof in a binder resin. When a release agent compatible with a binder resin is used, the release agent does not exhibit the release effect. To exhibit its release effect, release agents incompatible with a binder resin are preferably used to form domains of the release agents.
In addition, the dispersion state of a release agent as domain particles is also important. When a release agent is not well dispersed but agglomerated within a toner particle, the release agent does not have a sufficient release effect. Therefore, in order for a release agent to sufficiently exhibit its release effect, it is desired that the release agent be also locally dispersed close to the surface of a toner particle. However, when the ratio of the release agent present close to the surface of a toner particle increases, the release agent may protrude from the surface of the toner particle even when the release agent is fully dispersed. When the release agent protrudes from the surface of toner particles, the toner particles agglomerate, thereby reducing the fluidity of the toner particle. When a two component developer including a toner and a magnetic carrier is used, such release agents protruding from the surface of toner particles transfer to the magnetic carrier over a long period of use, resulting in deterioration of chargeability. Such release agents may transfer to an image bearing member, thereby causing filming thereon. Further, high temperature preservability of the toner deteriorates, meaning that the toner is not stable for a long period of time. In contrast, when the particle diameter of a release agent in a dispersion state is too small, the release agent is excessively and minutely dispersed, resulting in insufficiency of its release effect. Therefore, the content of a release agent added to a toner and its dispersion state therein are extremely important.
Japanese Patent No. 2663016 (hereinafter referred to as JPN) discloses a toner manufactured by suspension polymerization of a compound having a polar radical and a polymeric monomer having a release agent in an aqueous medium. The toner obtained can contain a release agent having a low melting point. A toner manufactured by a pulverization method cannot contain such a release agent. It is said in the literature that, unlike polar components, non-polar components such as a release agent do not exist locally in the proximity of the surface of a toner particle but take a pseudo capsule structure the surface of which is covered by polar components. However, distribution state of the release agent in the toner particle is not analyzed and unknown. JPN 3225889 discloses a toner which contains a release agent in an amount of 0.1 to 40% by weight. In addition, the weight ratio of the release agent protruding from the surface of the toner particle to all the components protruding therefrom is 1/100 to 10/100. The ratio of the release agent protruding from the surface of the toner particle is measured by Electron Spectroscopy for Chemical Analysis (ESCA) and determined. However, the analysis by ESCA is limited to the depth of around 0.1 μm from the outermost surface of a toner particle. Therefore, it is impossible to know the dispersion state of the release agent existing further inside of the toner particle which can suitably exhibit its release effect in the fixing process. Published unexamined Japanese Patent Application No. 2002-6541 discloses a toner encapsulating a release agent. The release agent is localized close to the surface of the toner particle. However, the detailed dispersion state of the release agent present close to the surface of the toner particle is unknown.
Because of these reasons, a need exists for a toner which can prevent the occurrence of offset even when a fixing roller on which no amount of or an extremely small amount of oil is applied is used, which can have excellent high temperature preservability when stored in an image forming apparatus for a long period of use and which can produce color images with suitable gloss and excellent color reproducibility.